1. Technical Field
The disclosed embodiments generally relate to systems and methods for detecting flaws in coated articles.
2. Description of the Related Art
In the process of electrophotographic imaging, a photoconductive member is electrically charged to a uniform potential. The charged member is exposed to a light image of the original document. The light selectively discharges areas on the surface, while leaving other areas uncharged, thus producing an electrostatic latent image. A developer material, typically containing charged toner particles with opposite polarity as that of the photoconductive member is brought into contact with the exposed photoconductive member. The charged toner particles are transferred to oppositely charged areas on the photoconductive member's surface to form a visible image. An electrostatically charged blank copy sheet is brought into contact with the photoconductive member containing the toner particles, and the toner particles are transferred to the copy sheet. The toner particle image on the blank copy sheet is then heated to permanently affix the toner particles to the sheet to form a “hard copy” image.
Electrophotographic imaging members are well known in the art. An electrophotographic drum is typically used in copiers and printers, and comprises an electrically conductive hollow cylindrical metal substrate in the form of a tube. Typically, the tubes are made from aluminum or other reflective material. To achieve the desired dimensional properties required for these devices, the aluminum tubes are often machined on a lathe and left with a specular or mirror surface, which produces congruent reflection upon exposure to radiation.
The electrophotographic drums of this nature are coated, typically with several layers of coating material, with at least one of which coating layers comprising an organic photoconductive (“OPC”) coating. These “layered photoreceptors” have at least a partially transparent photosensitive or photoconductive layer overlying a conductive ground plane, which typically is comprised of the machined mirrored aluminum tube. The layers may be single-layered or multi-layered, such as members having an inner layer of undercoat material and outer layer of change transport material. The tube may be rough or honed, and it may be made of other materials, such as other metals or conductive polymers.
Uniformity of the substrate surface and the coated substrate surface is critical for producing clear images in the electrophotographic process. Uniformity of the outermost transparent or at least partially transparent coating (collectively referred to herein as “transparent coating”) is particularly critical for color electrophotographic imaging. Typically, the transparent coating mean thickness may be about 20 μm to about 30 μm. The transparent coating can have coating thickness defects ranging from about 1 μm to about 30 μm. Submicron-sized defects are also possible, while larger defects are possible with thicker coatings.
Coating thickness defects can be in the form of “dimples” which have a coating thickness lower than the mean coating thickness, or “bumps” which have a thickness greater than the mean coating thickness. The coating defects may appear as circumferential banding. When visible electromagnetic radiation, or light, is impinged upon these coating defects at an oblique angle, there is little or no light scattering; the reflection from these coating defects is primarily specular, that is there is a mirror angle reflection. These coating defects in general are referred to as low frequency specular surface flaws due to the subtle nature of the change in coating thickness that accompanies these defects and to the mirror angle specular reflectance of light from these defects.
Low frequency specular surface flaws can be categorized by their thickness difference with respect to the mean coating thickness. For example, in a coating having a thickness of about 25 μm, flaws in the coating on the order of about 1 μm or less may be categorized as Level 0 (zero) flaws; flaws on the order of about 5 μm peak-to-peak (about 1.7 μm peak-to-reference, where reference is the nominal level of the exterior coating) categorized as Level 1 (one) flaws; flaws on the order of about 7.5 μm peak-to-peak (about 5 μm peak-to-reference) may be categorized as Level 2 flaws; and flaws on the order of about 21 μm peak-to-peak (about 18 μm peak-to-reference) may be categorized as Level 3 flaws.
Low frequency specular surface flaws detrimentally affect the performance of the OPC drum photoreceptor in reproducing images. Flaws as small as about 1 μm can have a detrimental effect on the reproduced image. As indicated, the flaws are areas of different coating thicknesses, and as such they have different charging and discharging properties as compared with the flawless areas of the coating and as compared with each other. This typically results in banding on the final image. This is even more critical in high speed color xerographic engines where color registration is critical for true color image reproduction.
Currently, machine vision inspection methods for detecting surface flaws, in general, include dark field angle, use of broad structured light, and laser profiling, for example as taught in U.S. Pat. No. 6,157,450, the disclosure of which is incorporated herein by reference in its entirety. These methods, however, have proved not to be useful in detecting low frequency specular surface flaws on coated substrates. Low frequency specular surface flaws of Level 3 or lower can only currently be observed by manual visual inspection. This method is tedious, inefficient, costly, and time consuming. A cost efficient, automated surface flaw detection means is needed.
Accordingly, there is a need for an improved apparatus and method for detecting low frequency specular surface flaws on coated substrates.